1. Field of the Art
This invention relates to an ultrasound transducer with an electronic focusing function.
2. Prior Art
Medical ultrasound examination systems are used mainly for acquiring information on conditions of patient""s body tissues or the like. Ultrasound examination systems are equipped with an ultrasound probe including an ultrasound transducer consisting of a piezoelectric transducer element or elements. The transducer element of this sort functions to transmit an ultrasound beam into a patient""s body and to receive echo signals from body tissues in various portions of a tomographic area or in positions of different acoustic impedances. The received return echo signals received by the transducer element are converted into electric signals, which are then transferred to an ultrasound sound signal processor to undergo various signal processing operations and thereby converted into video signals to be displayed on a viewing screen as ultrasound images. An ultrasound image is displayed by way of variations in luminosity on a large number of acoustic lines which are set on a viewing screen according to distances of scanned portions as converted from time durations to reception of return echo signals. The traveling or transmission speed of ultrasound waves in human body is almost same as transmission speed in water.
In order to improve the quality of ultrasound images, it is necessary to enhance the intensity of return echo signals. Further, for the purpose of increasing the range of penetration into patient""s body by an ultrasound beam, it is desirable that the output power of an ultrasound beam should be as large as possible. The ultrasound beam power is determined depending upon the active surface area of an ultrasound transducer. Namely, in order to obtain an ultrasound beam of higher output power, it becomes necessary to employ an ultrasound transducer with a broader active surface area. Generally, an ultrasound beam which is transmitted from a transducer element tends to spread or diverge in the travel direction, and, particularly in case of a transducer with a broad active surface, this tendency becomes very conspicuous to invite degradations in directional resolution. Besides, in receiving return echoes from various tomographic scan areas of different acoustic impedances, delays in signal reception time occurs in peripheral portions of the active surface as compared with center portions thereof. That is to say, delays in signal reception time phase constantly occur to an ultrasound transducer to increase the width of received pulse signals to a corresponding degree. These delays in reception time phase result in degradations in picture quality and sharpness of ultrasound images to be produced.
In this regard, it has been known and attempts have been made to provide an ultrasound transducer with an acoustic focusing function. Typical of the attempts in this particular aspect is the provision of an acoustic lens on the surface of an ultrasound transducer. Nevertheless, acoustic lenses are normally fixed in focal distance and therefore incapable of focusing to a position which is at a desired depth or of shifting a focus position over a certain range in depth in the manner as in the so-called dynamic focusing.
Ultrasound scan systems are generally categorized into a mechanical scan type and an electronic scan type. An electronic scan type ultrasound transducer unit is constituted by a large number of transducer elements which are arrayed in a predetermined direction. The arrayed transducer elements are sequentially driven in a certain timing to scan a predetermined range. The width of a transmitted ultrasound beam can be narrowed to a certain extent by controlling the timing in driving the respective transducer elements of the ultrasound transducer unit. This is the so-called electronic focusing. It is also possible to apply the electronic focusing to return echo signals. Namely, a delay circuit can be connected to each one of the transducer elements to adjust the delay time in signal reception independently of each other. In this case, the respective transducer elements can be operated in phase with each other in signal reception time by setting the delay time of each transducer element at an appropriate value according a difference in distance from a focus position to correct a delay in signal reception time.
In applying the above-mentioned electronic focusing, it is desirable to converge an ultrasound beam to a circular shape in section and to a minimal diameter and at the same time to bring the respective transducer elements exactly in phase with each other in signal reception time. For this purpose, a plural number of transducer elements of an ultrasound transducer unit are arranged in an annular array and in such concentric relations as to have different signal receiving positions from each other.
Regarding the piezoelectric material for the ultrasound transducer elements, it is the general practice to employ ceramic semiconductor such as titanium zirconate, barium titanate or the like. The ceramic semiconductor materials as just mentioned have a drawback that it is difficult to fabricate the transducer elements into a desired shape. However, it is necessary to form transducer elements into annular or arcuate shapes especially in case they are to be arranged in an annular array, despite the difficulties which are encountered in machining the ceramic semiconductor material precisely to predetermined surface curvatures. Due to high probabilities of fractures and cracking in a fabrication process, transducer elements of this sort are usually very expensive. Lately, in addition to the ceramic semiconductor materials as mentioned above, high polymer piezoelectric materials have been developed and put in use. Examples of the high polymer piezoelectric materials include PVDF (polyvinylidene fluoride), copolymer of VF (vinyl fluoride) and TrFE (trifluoroethylene) or TFE (tetrafluoroethylene), alternating copolymer P (VCDN/VAc) of VCDN (vinylidene cyanide) and Vac (vinyl acetate) and the like. The use of such a high polymer piezoelectric material may make it possible to arrange electrodes on piezoelectric elements substantially annularly and in such a way as to produce the same effects as an annular array without necessitating to machine the respective piezoelectric elements into an annular or arcuate shape. Nevertheless, as compared with the ceramic semiconductor type transducer elements, generally transducer elements of a high polymer type piezoelectric material are inferior in transmission efficiency and mechanical strength.
In view of the foregoing situations, it is an object of the present invention to provide an ultrasound transducer which can be fabricated in an advantageously facilitated manner and which is capable of electronically focusing an ultrasound beam to a desired position and satisfactory in signal transmission/reception efficiency.
It is another object of the present invention to provide an ultrasound transducer employing a polygonal array of transducer elements which are satisfactory in mechanical strength and can produce electronic focusing effects akin to an annular array.
It is still another object of the present invention to provide an ultrasound transducer having transducer elements of straight block-like forms arranged in a polygonal array which can operate substantially in the same manner as an annular array.
In accordance with the present invention, the above-stated objectives are achieved by the provision of an ultrasound transducer which basically comprises: an array of transducer elements arranged in a polygonal shape on a common plane, the array of transducer elements including a core unit consisting of a single transducer element of a polygonal shape and a plural number of frame units each consisting of a plural number of transducer elements arranged in a polygonal shape around the transducer element of the core unit, each one of the transducer elements of the frame units being formed in the shape of a straight square block and being electrically connected to adjacent transducer elements of the same frame unit but insulated from transducer elements of an adjacent frame unit through an insulating coat layer.
In this instance, preferably each one of the transducer elements of the core and frame units is constituted by a ceramic semiconductor type piezoelectric material which is superior in signal transmission efficiency and mechanical strength. Besides, the transducer elements of the core and frame units are so dimensioned as to have substantially the same active surface area.
The polygonal array does not have the respective transducer elements arranged on concentric circles. However, the transducer elements of the frame units are arranged to form closed polygonal loops around the transducer element of the core unit in the fashion of a pseudo annular array. The polygonal array approaches a circle in shape if the number of apexes is increased from tetragon to hexagon, from hexagon to octagon and so forth, but an increased number of apexes will make it more complicate to fabricate and assemble the transducer elements of the respective frame units. Accordingly, it is most preferable to arrange the frame units in an square or hexagonal array in view of closeness to an annular array in shape as a whole and easy fabrication and assembling of transducer elements. The transducer elements of the respective frame units can be fabricated easily in a facilitated manner because they are in the form of straight square blocks without any curved surfaces.
Each one of the transducer elements of the core and frame units should have an electrode on front and back sides thereof. The back side of each transducer elements can be connected to a common electrode. Accordingly, in arraying the respective transducer elements in a polygonal shape, the back side of each transducer element is placed on a common electrode sheet. On the other hand, a front electrode is laminated on the front side of each one of the transducer elements of the core and frame units.
The front electrodes on the front sides of the transducer elements of each frame unit should be connected with front electrodes of adjoining transducer elements. Namely, the respective transducer elements of the same frame unit are electrically connected with each other through conductive members, but should be insulated from transducer elements of an adjoining inner or outer frame unit. For this purpose, for instance, it is possible to provide an air gap or to interpose a sheet-like insulating member between transducer elements of inner and outer frame units. In this regard, from the standpoint of facilitating assembling work, it is preferable to provide insulating coat layers at least on opposing side surfaces of transducer elements of adjoining inner and outer frame units which face toward each other, permitting to locate the transducer elements of the two adjoining frame units closely to each other. For the necessity of drawing wires to the back side of the transducer elements of the core unit and the frame units, except the transducer elements of an outermost frame unit, through holes are normally required. However, for this purpose, it is advantageous to provide a notched portion at one corner of a transducer element in each frame unit. A notched portion of this sort can substantially serve as a through hole and provides a passage for drawing a wire from front electrodes of the transducer elements of each frame unit to the back side of the transducer.
The above and other objects, features and advantages of the present invention will become apparent from the following particular description, taken in conjunction with the accompanying drawings which show by way of example some preferred embodiments of the invention.